In wire electric discharge machining, an insulating machining solution is interposed between a wire electrode and workpiece. While the wire electrode and workpiece are being moved relatively with each other, electric power for machining is supplied from an electric power unit for machining to between the wire electrode and workpiece, so that the workpiece is machined by electric discharge energy.
Electric power units of the above conventional wire electric discharge machine used for wire electric discharge machining are disclosed, for example, in Japanese Unexamined Patent Publication Nos. 8-1438, 8-300222, 9-183019 and 7-290317. In the above electric power units for machining of the conventional wire electric discharge machine, a bipolar electric discharge inductive voltage is impressed upon between the wire electrode and workpiece so as to generate electric discharge, and then a main electric discharge current for machining is superimposed so as to conduct electric discharge. While importance is put only on the object of preventing the occurrence of electrolytic corrosion of the workpiece, the polarity is changed over to improve deviation of the polarity of electric discharge inductive voltage.
In the electric power unit for machining of the above conventional wire electric discharge machine, in order to improve deviation of the polarity, the polarity of bipolar voltage is changed over. Therefore, in the case of an abnormal state of electric discharge between the electrodes such as an abnormal state of concentrated electric discharge, it is necessary to reduce the speed of feeding the workpiece to be machined, or it is necessary to stop the feed of the workpiece, or it is necessary to retract the workpiece. Alternatively, it is necessary to increase the quiescent time of an impression voltage pulse. Therefore the electric discharge machining efficiency is lowered. Further, there is provided no means for continuing electric discharge in the case of normal electric discharge between the electrodes. Therefore, it is impossible to conduct electric discharge machining effectively.
Japanese Examined Patent Publication No. 3-119012 discloses an electric power unit for machining in which electric discharge is conducted as follows. In the case where an abnormal state of electric discharge is detected between the electrodes, electric discharge of reverse polarity is conducted while the workpiece is used as a negative electrode and the wire electrode is used as a positive electrode. In the case where a normal state of electric discharge is detected between the electrodes, electric discharge of straight polarity is conducted while the workpiece is used as a positive electrode and the wire electrode is used as a negative electrode. Whenever the case of an abnormal state of electric discharge, electric discharge machining of reverse polarity is conducted, so that the electric discharge machining speed is increased by utilizing electrolytic action of a machining solution.
In the electric power unit for machining of the above conventional wire electric discharge machine, the following problems may be encountered. In the case where an abnormal state of electric discharge between the electrodes is detected, electric discharge of reverse polarity is conducted. This abnormal state of electric discharge occurs in the case where the wire electrode and workpiece are short-circuited to each other or in the case of concentrated electric discharge. In the case of the occurrence of short circuit, even if voltage is impressed upon between the wire electrode and workpiece, no electric potential difference is generated. Therefore, no electrolytic action occurs. In the case of the occurrence of concentrated electric discharge, even if voltage is impressed upon between the electrodes, the electric potential difference is so small that an electrolytic action of the machining solution is not strong. Accordingly, even if the electrolytic action of the machining solution is utilized, the machining speed can not be increased sufficiently high. Further, it is difficult to early recover from the above abnormal state of electric discharge. Therefore, it is necessary to reduce the speed of feeding the workpiece to be machined, or it is necessary to stop the feed of the workpiece, or it is necessary to retract the workpiece. Alternatively, it is necessary to increase the quiescent time of an impression voltage pulse. Therefore the electric discharge machining efficiency is lowered.